It has been reported that using the Repair function (in the Programs and Features Control Panel) for Revit Server will cause Revit Server Administrator to stop working, showing the following message:
We have found that the repair process is changing the Managed Pipeline mode for the Revit Server Application Pool from Integrated to Classic, and this change is leading to the error above.
Fortunately fixing this can be fairly fast:
1. Open IIS Manager (Microsoft link).
2. Change the Managed Pipeline mode for the Revit Server AppPool
from Classic -> Integrated
Note: In the image above, Revit Server 2014 was repaired (through the Control Panel), and the Managed Pipeline Mode was incorrectly set to Classic.